Thermomechanical effects present a major challenge to developing a solid-state laser (SSL) for generation of high-average power (HAP) with near diffraction-limited beam quality (BQ). In particular, distortions to optical phase fronts caused by transverse temperature gradients within a SSL gain medium degrade beam quality (BQ) and render the output beam useless for many important applications. A class of SSL known as xe2x80x9cactive mirror amplifierxe2x80x9d (AMA) has shown effective reduction of transverse temperature gradients and demonstrated generation of laser output with very good BQ. A general configuration of a laser gain medium in an active mirror (amplifier) configuration is disclosed in the prior art illustration of FIG. 1.
The AMA was first disclosed by Almasi et al. in U.S. Pat. No. 3,631,362 (1971). In the original AMA concept, a large aperture (up to 25 cm in diameter), edge-suspended, Nd-Glass disk (or slab) is pumped by flashlamps and liquid-cooled on its back face. These devices were used in a large-scale, giant pulse laser amplifier chain (rather than a laser oscillator) operating in a low-average power mode at a very low repetition rate (typically one pulse per hour). See for example, J. Abate et al., xe2x80x9cActive Mirror: A Large-Aperture Medium Repetition Rate Nd:Glass Amplifier,xe2x80x9d Appl. Opt., vol. 20, no. 2, 351-361 (1981) and D. C. Brown et al., xe2x80x9cActive-Mirror Amplifier: Progress and Prospects,xe2x80x9d IEEE J. of Quant. Electr., vol. 17., no. 9, 1755-1765 (1981).
Brauch et al., in U.S. Pat. No. 5,553,088 (1996), discloses a variant of the AMA known as the xe2x80x9cthin disk laserxe2x80x9d. This device uses a diode-pumped gain medium disk with a small optical aperture, typically a few millimeters in diameter and 200-400 micrometers in thickness, soldered to a heat sink. See, for example, A. Giesen et al., xe2x80x9cScalable Concept For Diode-Pumped High-Power Lasers,xe2x80x9d Appl. Phys. B vol. 58, 365-372 (1994). The prior art disclosed a laser oscillator using one or more of such disks made of Yb:YAG gain media placed in a stable resonator configuration. These devices demonstrated laser outputs approaching 1 kW average power and with a BQ around twelve times the diffraction limit. See, for example, C. Stewen et al., xe2x80x9c1-kW CW Thin Disk Laser,xe2x80x9d IEEE J. of Selected Topics in Quant. Electr., vol. 6, no. 4, 650-657 (July/August 2000).
The applicant""s first co-pending patent application, U.S. Ser. No. 09/505,399, entitled xe2x80x9cActive Mirror Amplifier System and Method for a High-Average Power Laser Systemxe2x80x9d, which is hereby made a part hereof and incorporated herein by reference, discloses a new AMA concept suitable for operation at high-average power and good BQ. The invention uses a large-aperture solid-state laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 and 15 cm, mounted on a rigid, cooled substrate, and optically pumped by semiconductor diodes. Pump power is injected into the front or back face of the disk. The disk is attached to the substrate by a hydrostatic pressure differential between the surrounding atmosphere and the gas or liquid medium in the microchannels embedded in the substrate.
The applicant""s second co-pending patent application, U.S. Ser. No. 09/767,202, entitled xe2x80x9cSide-Pumped Active Mirror Solid-State Laser for High-Average Powerxe2x80x9d, which is hereby incorporated by reference, discloses a large aperture AMA wherein optical pump radiation is injected into the peripheral edge of a gain medium disk. Side-pumping takes advantage of the long absorption path (approximately the same dimension as the disk diameter), which permits doping the disk with a reduced concentration of lasant ions and provides a corresponding reduction in required pump radiation intensity.
The applicant""s third co-pending patent application, U.S. Ser. No. 09/782,788, entitled xe2x80x9cHigh-Average Power Active Mirror Solid-State Laser with Multiple Subaperturesxe2x80x9d, which is hereby incorporated by reference, discloses an AMA wherein a very large optical aperture is filled by multiple AMA subapertures. This co-pending patent application also discloses an AMA with the laser gain medium disk attached to the substrate by a diffusion bond rather than by hydrostatic pressure.
The teachings of co-pending patent applications Ser. Nos. 09/505,399, 09/767,202 and 09/782,788 provide numerous advantages over prior art solid-state lasers and allow generation of near diffraction limited laser output at very high average power from a relatively small device. In particular, analysis shows that an AMA module constructed in accordance with one or more of the above-referenced applications and using a Nd:GGG gain medium disk with a 15 cm diameter and 2.5 mm thickness can produce 15 kW of average laser power available for outcoupling with near diffraction limited BQ. See, for example, J. Vetrovec, xe2x80x9cActive Mirror Amplifier for High-Average Power,xe2x80x9d in SPIE vol. 4270, 2001. Co-pending application U.S. Ser. No. 09/505,399 discloses explicitly how multiple AMA modules may be used to construct a laser amplifier, especially as may be suitable for a laser configuration known as a master-oscillatorxe2x80x94power amplifier. There are, however, many important applications that would benefit from a HAP solid-state laser oscillator producing a near diffraction-limited BQ output beam.
A laser oscillator employs a laser gain medium inside an optical resonator of suitable configuration. Photons oscillating from one end of the resonator to the other end thereof constitute electromagnetic energy which forms an intense electromagnetic field. The shape of this field is precisely dependent not only upon the photon wavelength, but also upon the mirror alignment, curvature and spacing, as well as the optical aperture and inhomogenieties of the laser gain medium. This field can assume many different cross-sectional shapes, termed transverse electromagnetic modes (TEM), but only certain modes, or a mixture of them, are useful for utilizing the laser power.
In many laser applications, the most desirable mode is the fundamental mode (i.e., TEMoo, Gaussian, or diffraction-limited mode), which also has the smallest transverse dimensions of all modes. In a laser oscillator, to enable extraction of a near diffraction-limited beam from a large-aperture gain medium it is necessary to design a resonator which supports a large size TEMoo mode under operational conditions. While laser gain elements in an AMA configuration may appear to be natural candidates for construction of a HAP SSL oscillator producing a near diffraction-limited BQ output beam, numerous challenges must be overcome, including:
1. While a large optical aperture of the AMA gain medium is essential to generation of high laser power, its advantages would be wasted if the optical resonator of the laser oscillator could not support optical TEM large enough to fill the AMA aperture;
2. To obtain good BQ, it is necessary to design a resonator having good discrimination against higher order TEM;
3. Large transverse dimensions of the AMA aperture restrict the designer to a relatively low laser gain per AMA module, which exacerbates the problem of extracting available laser power from the AMA gain medium;
4. Low laser gain may also limit the resonator outcoupling fraction which, in turn, may lead to a reduced laser beam intensity in the far field;
5. While using an array of AMA modules for successive amplification of the beam enables the desired laser gain to be obtained, this requires a resonator capable of producing large TEM size over a long propagation path;
6. Using multiple AMA modules in a laser oscillator increases alignment sensitivity, which, in turn affects stability of oscillating TEM. Some alignment issues may be alleviated by using a stable and rigid alignment platform (optical bench), however; such a platform often represents a constraint to device integration into a compact, lightweight package.
7. During the startup, the AMA gain medium experiences a rise in temperature until temperature gradients for steady-state operation are developed. This situation further aggravates mode and alignment stability.
It should be noted that such problems are far less severe in the thin disk laser of the prior art, which employs a very small aperture gain element and generates modest average power with modest BQ. This permitted using the thin disk laser gain medium in a laser oscillator with a stable optical resonator. Using such a stable resonator is entirely inappropriate for use with a large aperture AMA gain medium.
One challenge associated with lasers employing a gain medium with a large optical aperture is designing a resonator supporting a low order (preferably TEMoo) mode(s) that can efficiently fill the entire aperture. Forty years of laser development has shown that obtaining high-average power output with good BQ from a solid-state laser with conventional stable resonators poses almost insurmountable problems. Such a stable laser resonator would require a cavity length that is either impracticably large, or would use an expansion telescope, or would have to be made in a folded configuration that increases the number of mirrors required. Stable resonators with long cavity or telescopic beam expanders are also very sensitive to mirror alignment and impractical for integration onto mobile platforms. For these reasons, laser oscillators for HAP are generally practiced with an unstable resonator, which have shown high efficiency for extracting available power associated with the cavity mode in a near diffraction-limited beam. Such a near diffraction limited beam provides a near optimum distribution of radiant energy in the far field as is required for many important applications. Unstable resonators are often practiced in a confocal configuration with either a positive or negative branch variants. For additional information, see for example, A. Siegman, xe2x80x9cLasersxe2x80x9d, John Willey and Sons, New York, N.Y., 1985.
Ring unstable resonators (introduced by Buczek et al. in U.S. Pat. No. 3,824,487), in contrast to linear unstable resonators, provide much increased design flexibility and a number of design possibilities over and above the advantages possessed by ring resonators for laser applications generally. A ring unstable resonator can be designed, for example, to have a short telescopic magnification (i.e., beam expansion) section using conveniently available optical elements with short radii of curvature, and then to have much longer collimated regions through the laser gain medium. Negative-branch ring resonators can also be built with spatial filters, which can cleanup the mode patterns and filter out some of the phase distortion effects caused by inhomogeneous elements in the resonator. Ring resonators also offer the possibility of unidirectional oscillation (traveling wave), which eliminates spatial hole burning effects found in linear resonators and which results from interference between counter-propagating optical waves. Ring resonators are often designed using various sorts of folded sections in order to achieve near normal incidence on at least some of the mirrors, since this minimizes astigmatism from curved mirrors and permits standard coatings to be used. It is also possible to design a negative-branch ring unstable resonator such that each round trip corresponds to an image relay which images a magnified version of the coupling aperture back onto itself each round trip. Such a self-imaging configuration is known to yield a particularly smooth and uniform lowest order mode pattern in an unstable resonator. For additional information see, for example, the above noted publication by Siegman, or N. Hodgson and H. Weber, xe2x80x9cOptical Resonatorsxe2x80x9d, Springer-Verlag, London, 1997.
AMA modules disclosed by the applicant in the above noted U.S. patent applications provide a laser gain medium with very high homogeneity over a large aperture and under operational conditions. However, when a large number of AMA modules are used to construct a laser oscillator, even small residual inhomogenieties experienced by an optical wave recirculating inside a resonator may add up to significantly perturb the optical phase front. Furthermore, in all operational unstable resonator devices, there are various naturally occurring sources of phase and amplitude distortions that degrade both the intracavity mode and the resultant far-field irradiance structure. One effective approach to addressing this problem is to use an intracavity adaptive optic element(s) to drive the aberrated mode structure back towards the ideal unaberrated mode of the resonator. The term xe2x80x9caberrationsxe2x80x9d as used herein refers to distortions of the optical wavefront from flat or simple curvature conditions.
A conventional adaptive optics system generally includes a deformable mirror whose surface can be deformed selectively by means of actuators. Suitable deformable mirrors have been disclosed in the prior art. See, for example, J. E. Pearson and R. H. Freeman, in Applied Optics, vol. 21, page 4 (1982). The deformation of the mirror is typically within the range of several wavelengths of the impinging laser light. As the incoming aberrated light strikes the deformable mirror, it is reflected from the mirror such that the mirror compensates, at least partially, for the aberrations. The reflected light impinges upon a beam splitter or a sampler that directs a small fraction of the laser beam to a wavefront sensor. The sampled signals are transmitted to a controller, which drives the actuators of the deformable mirror, in a feedback loop, in response to the sampled signals. This compensates for the aberrations in the light wave. Such a system for intracavity wavefront correction was first disclosed by Frieberg in U.S. Pat. No. 4,249,140 (1981).
However, the wavefront can also be tilted and will thus move in the wrong direction. In many adaptive optics systems it would not be desirable to compensate for tilt using a deformable mirror, since the magnitude of the tilt might be much greater than the range of the deformable mirror, and consequently, the tilt would not be removable. For this reason, modern adaptive optics systems also employ a steering mirror for continuous compensation of tilt in the wavefront direction. A deformable mirror and a steering mirror may each have their own sensor or use a common sensor. The actions of the two components are separated by removing any tilt portion from the wavefront measurement used to drive the deformable mirror and using it to drive the steering mirror. An example of an adaptive optics system employing both a steering mirror and a deformable mirror was disclosed by Salmon in U.S. Pat. No. 5,745,309 (1998). For additional pertinent information on adaptive optics, see for example Chapter 3, Intracavity Laser Beam Control and Formation in xe2x80x9cLaser Resonators: Novel Designs and Development,xe2x80x9d by A. Kudryashov and H. Weber, SPIE Optical Engineering Press, Bellingham, Wash. (1999) or R. K. Tyson, xe2x80x9cPrinciples of Adaptive Optics,xe2x80x9d Academic Press, San Diego, Calif. (1998).
A principal object of the present invention is to provide a SSL oscillator with a large-aperture gain medium in an AMA configuration capable of producing high-average power output with good BQ. In particular, the present invention meets a number of significant needs including, but not limited to, the following:
A resonator supporting large size, low-order laser TEM;
A resonator with efficient mode discrimination against higher order TEM;
A high resonator outcoupling for good BQ;
A collimated intracavity beam;
A means to prevent spatial hole burning;
Intracavity laser TEM control by adaptive optics;
A means for optical wavelength tuning;
An axisymmetric arrangement of AMA modules for compact integration;
An axisymmetric optical bench for stable and compact alignment platform;
An alignment control and beam jitter rejection by steering mirror;
Kinetic mounting of AMA modules; and
A pressure balanced means for connecting coolant lines to AMA modules.
A first preferred embodiment of the present invention comprises a SSL with an array of AMA modules placed in a linear unstable resonator that provides a large fundamental mode size, excellent transverse mode control, and collimated output beam. A second preferred embodiment of the present invention comprises a SSL with an array of AMA modules placed in a ring unstable resonator that provides much increased design flexibility and an increased number of design possibilities, longer collimated beam regions, and which avoids spatial hole burning. A third preferred embodiment of the present invention provides additional improvements in laser power extraction from an AMA laser gain medium. The fourth and fifth preferred embodiments of the present invention provide axisymmetric arrangement of AMA modules for compact packaging. A sixth alternative preferred embodiment of the present invention provides AMA modules with an axisymmetric arrangement of gain elements for improved packaging and integration. Means for kinematically mounting AMA modules and supplying them with coolant by pressure balanced means are also disclosed.